Location awareness is typically an important aspect of mobile computing. Location awareness of nodes in a mobile communications network may also improve the overall utilization of that network. This is true both for mobile cellular networks as well as for mobile ad hoc topologies. Location awareness may also be used to enhance Travel Information Systems, which may lead to better utilization of the road network.
Traditional location techniques, like the Global Positioning System (“GPS”), typically deliver very accurate location information, but considerations of cost, size, form factor and power requirements currently make mass deployment too costly and impractical. Moreover, reliable GPS signals may be unavailable in many areas due to lack of line-of-sight with a required number of geosynchronous GPS satellites in, e.g., indoor locations, tunnels, subway passages, locations surrounded by tall buildings such as skyscrapers, or other areas where interference disrupts the signal. Further, as more systems become dependent on GPS (e.g., road toll systems) an alternate robust backup and/or referencing system would be advantageous. Uptake on telecom network-based location-determination techniques has also been quite low. Operators cite the high deployment and operational cost as the main inhibitors.
Cellular networks, like the Global System for Mobile Communications (GSM), are designed in such a way that each transmitter, i.e., base station, covers a specific geographic area (the “cell”). Typically, the cell radius may vary between approximately 0.1 and approximately 7.5 kilometers (km). The base station of each cell typically transmits a three part identifier, which includes a country code, a network or operator code, and a cell number (its “cell-id”). The combination of these three items provides a worldwide unique identifier for a cell. Other networks (e.g., various cellular or wireless networks) may utilize different identifier(s) for each uniquely identifiable cell that are also applicable to embodiments of the present invention. For example, a Wi-Fi or wireless local area network (WLAN) may utilize unique hardware-based identifiers typically termed Media Access Control (MAC) addresses.
A GSM device, e.g., a cellular phone, connected to the network will continuously monitor what it perceives to be the cell site with the strongest signal. Typically, this will be the signal from the nearest tower, but due to signal scatter or other interference this may also be one that is further away. Signal scatter may be related to variables like weather and traffic intensity, and is therefore not constant over time. Thus, the measured signal may also vary over time. The network device also typically monitors the relative signal strength it receives (“SSID”). There is a theoretical relation between the SSID and distance to the base station, but in practice this relation is not reliable enough to form the basis for a robust and accurate location determination system.